Biosurfactants (hereinafter, also referred to as BS), which are surfactants derived from living organisms, have high biodegradability and safety, and are expected to be used industrially as next-generation surfactants.
Sophorolipid, which is known as a glycolipid type BS, is a fermentation product obtained from fermentation by yeast. Sophorolipid can be easily produced by, for example, inoculating yeast on a liquid medium containing carbon sources, such as vegetable oil and fat, and sugars such as glucose, and stirring the medium while aerating the medium at a mild temperature and under pressure. Compared to other BSs, sophorolipid can be obtained with high productivity (e.g., about 100 g/L), and has been industrially used (Non-Patent Literature 1).
However, in the fermentation generation process of sophorolipid, since fermentation by-products (various organic acids and salts thereof, pigments, etc.) are also simultaneously produced, partially purified or partially extracted sophorolipid obtained after the end of fermentation emits specific odors. Therefore, there have been limits and many problems that needed to be solved in order to apply sophorolipid to drugs, quasi drugs, food products, and cosmetics.
For example, quasi drugs and cosmetics used on the body include rinse-off type products, such as shampoos (body shampoo, hair shampoo) and hair rinses, which are applied to the skin and then washed away, and leave-on type products, such as face lotions and milky lotions, which are applied and maintained on the skin. Since the pH of the skin is weakly acidic, weakly acidic products of both types minimally affect the skin, and market needs for such products are high. However, since hitherto known sophorolipids contain a large amount of various organic acids derived from fermentation, weakly acidic cosmetics and the like having such sophorolipids blended therein emit characteristic odors, and are not fit for use. These odors originate from lower fatty acids, such as acetic acid, butyric acid, and isovaleric acid, and there are techniques to reduce the odors through masking methods, etc. However, since the olfactory threshold (the minimum concentration of odor that can be sensed) for lower fatty acids is extremely low, and lower fatty acids have a high skin persistency due to having low volatility, an unpleasant acid odor remains over a long period of time.
In recent years, coenzyme Q10 and hyaluronic acid have been widely used in food products and cosmetics, and many studies have been conducted thereon since they have various bioactivities. At present, they are blended in many products to utilize their function. On the other hand, although sophorolipid, which is also a natural product, has been found to have unique functions, such as controlling emulsification power and percutaneous absorption of active ingredients (Patent Literature 1), actual usage examples thereof in external use compositions, such as cosmetics, are nonexistent.
The reason why there has not been much progress in the industrial use of sophorolipid, even though it is a material derived from living organisms, has high capability, and has high biodegradability and safety, is no other than the lack of a deodorization or impurity removing method on an industrial scale.
Generally, a purification process is the most difficult and costly process in producing a fermentation product. The reasons for this include the amount of fermentation products obtained from fermentation being remarkably small, for example, not more than 1 mass % per 100 mass % of liquid obtained at the end of fermentation. Thus, in a situation where a useful substance that has been produced through fermentation is extremely diluted and diffused in a liquid obtained at the end of fermentation, it is necessary to selectively extract or concentrate, etc., the intended useful substance. Sophorolipid, which is the target of the present invention, is no exception even though the productivity thereof is high.
For the purification of sophorolipid, many of the hitherto reported methods involve extraction by adding, to a liquid culture medium, an equivalent amount of hexane and ethyl acetate (e.g., Non-Patent Literature 2). However, the sophorolipid obtained from such methods still has characteristic odors. This is because odor components have properties chemically similar to sophorolipid, and are extracted in a manner similar to sophorolipid.
There is also a report of a method for purifying sophorolipid as a white substance (Non-Patent Literature 3). Here, a liquid culture medium itself is lyophilized, ethyl acetate is added to the dried object, the mixture is stirred at 30° C. for 2 days, ethyl acetate is distilled off, and a crystal is formed using hexane. However, in this method, since it is necessary to add a flammable organic solvent and leave it over several days, it is difficult to put the method to practical use. In addition, the sophorolipid obtained from this method still has slight odors.
Furthermore, with the method described above, it is necessary to recover or remove the organic solvent from a collected liquid. Therefore, energy and special equipment are required for treating waste liquid containing the organic solvent, resulting in an increase in cost. In addition, such usage of organic solvents requires strict management thereof from a standpoint of environmental impact and adverse health effects. Furthermore, when there is a possibility of an organic solvent remaining in the obtained sophorolipid, it becomes undesirable for application to food products and cosmetics. Thus, hitherto known methods of extraction, isolation, and purification are proposed merely from a standpoint of basic research, and industrial application is not taken into consideration.
From the standpoint of industrial application, purification has to be a process that is cheap and safe. When it is a widely used chemical product, the cost aspect becomes particularly important. Furthermore, at present, in addition to the biodegradability of a used product, it is important to establish a safer manufacturing process including the raw materials from a standpoint of LCA (Life Cycle Assessment). Therefore, for a sophorolipid that is derived from living organisms and is safe for a living body, it is also preferable to establish a production method without discharging or using hazardous organic solvents.
Therefore, establishment of a method for producing highly pure acid-form sophorolipid stably, cheaply, and with a high yield without using hazardous organic solvents is expected to dramatically advance the industrial application of sophorolipid as a new material that is derived from living organisms, is safe, and has excellent biodegradability.